Conventionally, after corona charging a single imagewise exposure of a photoconductor element, the latent image produced is developed into a visible toned image that is then electrostatically transferred to a receiver sheet and heat fused thereto.
It has been found that a plurality of high quality toned images cannot be produced from such a single imagewise exposure by repeating the subsequent step sequence of toner development, electrostatic transfer, and heat fusion.
A latent image can be produced within the photoconductor layer that is not erased when the layer is subsequently exposed to uniform overall light, if a suitably charged photoconductor element at the time of imagewise light exposure thereof is simultaneously subjected to corona charging, (see, for example, U.S. Pat. Nos. 4,063,943; 4,071,361; 4,297,423; and 4,442,191). When this procedure is followed, it is found that the photoconductor stores the latent image. Thus, multiple electrophotographic copies can be made using the known step sequence of toner development, electrostatic transfer, and heat fusion.
However, when this procedure is followed to produce a recorded latent image in a photoconductor element, successive copies of the image display increasingly blurred images. The latent image blurring is caused by image spreading in the photoconductor element.
It is presently theorized (and there is no intent herein to be bound by theory) that the reason for blurring is that nonuniform electric fields exist in the photoconductor element that cause the charge carrier therein to move both towards the free surface, to neutralize the surface charge, as well as laterally, to cause image spreading. Under uniform light exposure through the photoconductor element support, and for photogeneration of charge carriers near the edge of a character or line in an image, electrons and holes move laterally leading to the image spreading. Away from these edges, the electric fields are more uniform and the holes and electrons move perpendicularly to the film surface. If the uniform overall exposure is continued for a sufficiently long time period, the entire interface in the region between the photoconductive layer and the conductive layer will be driven to equipotential. However, if the uniform overall exposure is absorbed near the free (or imaged) surface, then no horizontal field exists, and hence no lateral image spreading occurs.
So far as now known, no photoconductor element is capable of being used in this process without the occurrence of the blurred image phenomenon during efforts to make multiple electrophotographic copies.